Methods for packaging light emitting devices and related microelectronic devices

ABSTRACT

A method for forming a light emitting device includes providing a light emitting diode (LED) configured to emit light of a first color and providing a plurality of semi-spherical lenses made of a silicone material that contains no phosphor material. Each of the lenses has a layer of phosphor material attached thereto. The method also includes testing the plurality of lenses to select a subset of lenses that converts light of the first color to light of a second color. The method further includes forming the light emitting device using the LED, one of the selected subset of lenses, and a heat conductive substrate. In an embodiment, after the testing of the plurality of lenses, one of the selected subset of lenses is disposed overlying the LED. In another embodiment, the testing of the plurality of lenses is conducted with a light source other than the LED.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority from and is a Divisionof application Ser. No. 12/977,248 filed on Dec. 23, 2010, the entirecontents of which are incorporated herein by reference.

This application claims the benefit of priority from and is aNon-Provisional application of Provisional application Ser. No.61/284,814 filed on Dec. 26, 2009, the entire contents of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to light emitting diodes, and, more particularly,to methods for packaging light emitting devices and relatedmicroelectronic devices.

2. Description of Related Art

Introduced as an electronic element in 1962, a light emitting diode(LED) may be used as indicator lamps in many devices, and used forlighting. Modern LEDs emit visible, ultraviolet and infraredwavelengths, with very high brightness.

LEDs are getting to replace automotive lighting (particularly brakelamps and turn signals) as well as traffic signals. With the advantagesof compact size, narrow bandwidth, high switching speed, and improvedreliability, LEDs are becoming more and more popular in the art.

SUMMARY OF THE INVENTION

Embodiments of this invention relate to semiconductor light emittingdevices (LED) as well as associated passive and active devices that areintegrated part of a complete LED system, and fabricating methodstherefore. More particularly, some embodiments of the present inventionprovide packaging and packaging methodologies for semiconductor lightemitting devices. Certain embodiments of the present invention describemethods of packaging LED, wherein LED chips are placed on metal heatsink for thermal dissipation, alternate material and method for itsattachment, where phosphor encapsulation structure is mounted in theassembly for light conversion. Further, in some embodiments, an opticalcomponent which is typically used as secondary optics for LED lightingmodule may be integrated into the package according to some ofembodiments of the present invention. This LED package platform hashigh-efficiency thermal dissipation, and substantially reduces cost inpackaging as well by eliminating several packaging steps as conducted intraditional methods.

In an embodiment, a method for forming a light emitting device includesproviding a light emitting diode (LED) configured to emit light of afirst color and providing a plurality of semi-spherical lenses made of asilicone material that contains no phosphor material. Each of the lenseshas a layer of phosphor material attached thereto. The method alsoincludes testing the plurality of lenses to select a subset of lensesthat converts light of the first color to light of a second color. Themethod further includes forming the light emitting device using saidLED, one of the selected subset of lenses, and a heat conductivesubstrate. In an embodiment, the lenses may be selected according to aspecific wavelength range of the second color. In an embodiment, afterthe testing of the plurality of lenses, one of the selected subset oflenses is disposed overlying said LED. In another embodiment, thetesting of the plurality of lenses is conducted with a light sourceother than said LED.

In another embodiment, a light emitting device includes a heatconductive substrate, at least one semiconductor light emitting diode(LED), and an encapsulant overlying the at least one LED. Theencapsulant includes a substantially uniform layer ofwavelength-converting material. Moreover, the encapsulant is preformedand pre-characterized for optical properties prior to being disposedoverlying the LED. In an embodiment, the encapsulant may be testedaccording to a specification for converting light of the first color tolight of a second color. In an embodiment, the encapsulant may beselected according to a specific wavelength range of the second color.In another embodiment, the substantially uniform layer ofwavelength-converting material is preformed into the encapsulant, e.g. alens.

These and other features and advantages of embodiments of the presentinvention will be more fully understood and appreciated uponconsideration of the detailed description of the preferredimplementations of the embodiments, in conjunction with the followingdrawings.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A and 1B are simplified cross-sectional views of LED packageswherein LEDs are placed on a metal heat sink, and phosphor encapsulationstructure is mounted for light conversion;

FIGS. 2A and 2B are simplified cross-sectional views of LED packageswherein LEDs are placed on a metal heat sink, and an array of phosphorencapsulation structures are mounted for light conversion;

FIG. 3 illustrates simplified cross-sectional views of different typesof LED phosphor encapsulation structures that may be attached on an LEDpackage according to embodiments of the present invention;

FIG. 4 is simplified cross-sectional views of LED package wherein LED isplaced on a metal heat sink, and an integrated reflector with heat sink.The reflector may have highly reflective coating to increase lightextraction through phosphor encapsulation structures; and

FIG. 5 is simplified cross-sectional views of LED package wherein LED isvertical type of chip, mounted on an insulating layer placed on metalheat sink, and a phosphor encapsulation structure is attached for lightconversion.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Embodiments of the present invention describe a new LED package platformto address issues of thermal dissipation, and provide significant costreduction for semiconductor light emitting devices. As used herein, theterm semiconductor light emitting device may include a light emittingdiode, laser diode, or/and other semiconductor device which may includeone or more semiconductor layers, and which may include silicone,sapphire, silicon carbide, or/and other semiconductor material. In thedescription below, an LED is used as an example to illustrate details ofpackaging methods.

In current LED packaging methods, LED chip is assembled into a carrier(level-1) such as ceramic, silicone or plastic. The level-1 package isusually necessary for phosphor process for color conversion, and colorbinning of the white color points. Next, a set of level-1 LED productsare integrated on PCB (level-2), and then mounted on a heat sink such ascopper, aluminum or other thermal conductive materials for thermaldissipation of the light engine.

In such packaging methods as the above described, there exists severalthermal barriers (interfaces like chip-carrier, carrier-PCB, andPCB-heat sink) to dissipate the heat from LED chip to heat sink in thedesign of LED light engine. Although several different package conceptswere proposed to resolve the thermal issues, these methods usually addcost to package. One of solutions is so-called Chip-on-board (COB),wherein LED chip is directly mounted on circuit board to reduce thermalbarriers and packaging cost. Although COB technique simplifies assemblyprocess, skipping level-1 process, however, it is known that poorthermal conductivity of PCB material limits LED application in highpower regime. In addition, dispensing phosphor-silicone mixture used forwhite light conversion in COB package has difficulties in achieving goodcolor quality such as color consistency, color uniformity over angle,color rendering index, and so on—where product optimization throughcolor binning of the white color points is not possible.

Features of new package platforms provided in embodiments of the presentinvention described below, which can be applied to LED lamp packageor/and light engine structure, include the following:

LED chip is mounted on metal heat sink to maximize capacity of heatdissipation.

Different mounting technique, where conventional conductive attachmentis accompanied by heat pipe—therefore increasing the thermal dissipationand reducing total thermal resistance by an order of magnitude.

Eliminating traditional level-1 and level-2 packaging gives costadvantage.

Integrate secondary optics with heat sink structure. The surface ofoptics may have highly reflective coating material.

Pre-characterized phosphor encapsulations such as phosphor lens orphosphor sheet can be attached on the packaging to significantly improvecolor quality such as color consistency, color uniformity, colorrendering, and so on, for the packaging structure.

The phosphor encapsulation such as lens may be attached individually tothe package or attached in an array to the light engine.

Attachment procedure to overlay phosphor layer over individually orarrayed wire bonded LEDs to make the light engine.

In some embodiments, an LED chip is directly mounted on a heat sink,which can be made of a material having suitable heat conductivity, forexample, a metal or other conductors. In other embodiments, aninsulating layer having suitable heat conductivity can be placed betweenthe LED die and the heat sink. Such an insulating layer can allow moreflexible wiring connections. In some embodiments, electrical circuitconnections can be formed on a circuit board attached on the heat sink.In an example, the circuit board can be placed adjacent to the LED chip.In another example, the LED chip can be placed in an opening of acircuit board.

In some embodiments, the LED chip is directly mounted on a heat sink andintegrated with a phosphor encapsulation structure. In an embodiment,the phosphor encapsulation structure can be a lens with aphosphor-containing light conversion layer built in. In someembodiments, the phosphor encapsulation structures can bepre-characterized and pre-sorted. Some examples of the phosphorencapsulation structures are described in our previous patentapplications. For example, U.S. Patent Application No. 61/216,374 on May15, 2009 and U.S. Patent Application No. 61/273,129 on Jul. 30, 2009.

Some alternative embodiments provide a lighting apparatus that includesan LED die configured to emit light within a first wavelength range anda phosphor encapsulation structure overlying the LED die. In anembodiment, the phosphor encapsulation structure is configured totransmit light of target color when receiving input light within thefirst wavelength range of LED die. The phosphor encapsulation structurecontains phosphor material which is pre-measured for color point tomatch specific wavelength of LED for desired white color point.

In some embodiments, the LED chips can be pre-tested and pre-binned, andthe phosphor encapsulation structure can also be pre-binned. Thenmatching LED chips and phosphor encapsulation structure can be selectedto form a lighting device for emitting certain target light color.

In some embodiments, the LED die may have two metal contacts on the sameside of the die or opposite sides of the LED die. For LED with metalcontacts on opposite sides, an insulating layer is necessary to connectthe LED's in series applications.

In some embodiments, a flat bottom phosphor encapsulation is used. Theloop of wire bond is lower than the height of circuit board for notdamaging wire during attachment. In an embodiment, the flat bottomphosphor encapsulation can be mounted on the edge of a circuit board.

Embodiments of the present invention will be now described below withreference to various examples illustrated in the figures.

FIG. 1A illustrates a light lamp or light engine structure, in where inLED 101 is placed on the top of a metal heat sink 301 with an epoxy orsolder material. Because LEDs are directly placed on a metal heat sink,this structure gives best-achievable thermal conductivity for LED lampor LED light engine. The heat sink may be one of any materials commonlyadapted in LED packaging such as copper, aluminum, ceramic, and so on. Alayer of printed circuit board (PCB) 201 is laminated on the top of heatsink as an insulating layer for electrical connection. The thickness ofPCB material is higher than the loop of wire bond, so that phosphorencapsulation can be placed on the top without damaging the bonded wire202. The space between LED chip and encapsulation is filled up with apredetermined amount of silicone gel. Alternatively, the space may befilled in with silicone material prior to phosphor encapsulation or lensattachment.

The phosphor encapsulation structure 102 containing phosphor particles103 may be a lens or a phosphor sheet as shown in FIG. 3. The colorproperties such as correlated color temperature (CCT) or color points ofphosphor encapsulation is pre-measured and selected to match LED fordesired color properties of LED lamps and light engines.

FIG. 1B illustrated a light lamp or light engine structure with aninclusion of recess in heat sink 301. The recess in the heat sinkincludes a surface 302 which may serve as a light reflector to increaseamount of blue photons for white light conversion in phosphorencapsulation structure 102. The reflector surface 302 may contain ahighly reflective coating material for example but not limited to suchas TiOx coating or whitish PCB material.

FIG. 2A and 2B illustrates a light lamp or light engine structures asdescribed in FIGS. 1A and 1B. In this embodiment, phosphor encapsulationstructure 104 is an array structure which is aligned on LED chips placedon the heat sink. Each encapsulation is still connected withencapsulation material which may have or may not have phosphor particlesin the connection 105. Keeping phosphor encapsulation structure 104 inan array form can simplify process steps during formation of phosphorencapsulation, as well as increase throughput of encapsulation placementin manufacturing.

As shown in FIG. 3, methods of forming phosphor encapsulation structureare described in our previous patent applications. The phosphorencapsulation structures mounted on heat sink may be in various shapesor in a sheet form depending on desired light quality such as coloruniformity or light radiation pattern. Materials of phosphorencapsulation structure may be epoxy or silicone material, orthermoplastic or thermosetting material, or ceramic plate, or glass orany materials that can be used for LED encapsulation. Phosphor particles103 may contain single or multi-layers of phosphors which may havedifferent optical properties depending on desired optical properties.

FIG. 4 illustrates a method of packaging LED in one embodiment of thepresent invention. LED 101 is placed on a heat sink 301 with an epoxy orsolders material 203. A reflector 303 is mounted on heat sink 301 withsolder material 305. Alternatively the reflector 303 may be formedtogether with heat sink 301 to eliminate thermal barrier of soldermaterial 305. The heat sink material may be copper, aluminum, ceramic,silicon, or other thermal conductive materials commonly used for LEDpackage. The height of reflector may or may not be greater than the loopof bonded wire 202, ensuring that the wires 202 are not damaged duringencapsulation placement. The surface of reflector 303 may be coated witha layer of highly reflective material 304, such as TiO_(x) or whitishPCB material to increase light extraction. PCB material 201 is attachedand may or may not be laminated on the surface of heat sink as aninsulator for electrical connection. The phosphor encapsulation 102 asthe above described may be in various shapes or a sheet as shown in FIG.3. A predetermined amount of silicone gel is injected to fill up thespace 106 after encapsulation placement. Alternatively, the space 106may be filled with silicone material prior to phosphor encapsulation orlens attachment. Alternatively, the space 106 may also be the bottompart of over layered 103 which fills up 106 with silicone material priorto lens attachment. The packaging structure may contain one of more LEDscovered with single phosphor encapsulation.

For LED chips with two metal contacts (p and n contacts) located on thesame side, LEDs may be connected in series or in parallel in the packageas the-above illustrated. The electrical connection can be proper layouton laminated PCB 201 for connection in series or parallel depending onapplication.

For LED chips with two metal contacts (p and n contacts) located on theopposite side so called vertical chip, LED mounted on metal heat sinkwhich serves as common ground for LED chips, results in parallelconnection. Therefore, an insulating layer 204 is required in betweenLED bottom contact, and the metal heat sink 301 for series connection ofvertical chips, as illustrated in FIG. 5. The electrical connection canbe proper layout on laminated PCB 201 for electrical connection. Otherthan insulating layer 204, all embodiments described in the above forLEDs with the metal contact on the same side in FIGS. 1 to 4 can beapplied to the vertical chip as well.

The insulating layer 204 may include metal bond pads 204 a on AlO_(x)204 b/aluminum 204 c, or SiO₂ 204 b/silicone 204 c, or ceramic, or PCBmaterial or other insulating materials with good thermal conductivity.In some embodiments of the present invention, material 204 b may be thesame material as heat sink 301 such as Aluminum, copper, ceramic,silicon or other thermal conductive materials commonly used for LEDpackage.

While the preferred embodiments of the invention have been illustratedand described, it will be clear that the invention is not limited tothese embodiments only. Numerous modifications, changes, variations,substitutions and equivalents will be apparent to those skilled in theart without departing from the spirit and scope of the invention.

1. A light emitting device, comprising a heat conductive substrate; atleast one semiconductor light emitting diode (LED); and an encapsulantoverlying the at least one LED, the encapsulant comprising asubstantially uniform layer of wavelength-converting material, whereinthe encapsulant is preformed and pre-characterized for opticalproperties prior to being disposed overlying the LED.
 2. The lightemitting device of claim 1, wherein the wavelength-converting materialcomprises LED phosphor.
 3. The light emitting device of claim 1, whereinthe encapsulant comprises LED lens.
 4. The light emitting device ofclaim 1, wherein the heat conductive substrate comprises an electricalcircuit for wiring connection.
 5. The light emitting device of claim 1,wherein the heat conductive substrate further comprises an integratedoptical reflector being shaped with part of the heat conductivesubstrate.
 6. The light emitting device of claim 1, further comprising abond-wire loop that does not exceed the very top surface of a circuitboard on the heat conductive substrate.
 7. The light emitting device ofclaim 1, wherein the encapsulant comprises a semi-spherical surface anda flat surface, and the layer of light-conversion material is attachedto the flat surface.
 8. The light emitting device of claim 1, whereinthe encapsulant comprises a semi-spherical surface and a flat surface,and the layer of light-conversion material is attached to thesemi-spherical surface.
 9. The light emitting device of claim 1, whereinthe encapsulant comprises a convex surface and a concave surface, andthe layer of light-conversion material is attached to the convexsurface.
 10. The light emitting device of claim 1, wherein theencapsulant comprises a convex surface and a concave surface, and thelayer of light-conversion material is attached to the concave surface.